The invention relates to a hip joint prosthesis comprising a shaft-like section insertable into a femur and a proximally arranged end cap assembly closing the a hollow space of the hip joint prosthesis.
Such hip joint prostheses are known in the field of endoprosthetics, for example, from DE 295 06 036 U1.
Problematic with such inelastic, tissue-replacing implants is, however, the interference with biological recovery, primarily due to the loss of blood vessels and nerves. In addition, the biomechanical quality of the support structure will suffer with increasing implantation time due to the partial elimination of its function. With the loss of biological control, however, the risk of infection by drug-resistant bacteria (MRSA=multi-resistant staphylococcus aureus) will increase. It has been shown that these bacteria may colonise the surface of metal implants in the form of an adherent bio film and resist attack from antibiotics with the aid of a mucous layer of polysaccharides.
Within the framework of orthopedic surgery, these problems can be addressed by magnetically induced electro-osteotherapy. Examples are known from DE 10 2004 024 473 A1 and DE 199 28 449 A1. In electro-osteotherapy, electric alternating potentials with a low frequency are induced in endoprostheses or osteosythesis means by subjecting an affected part of the body to an alternating magnetic field. It has long been demonstrated in numerous clinical applications of the technology according to the method to chronically therapy-resistant, in most cases, infected bone defects, cysts and tumor metastases, as well as in clinic-oriented experimental studies that an optimum healing effect can be obtained by using implants as sources of extremely low-frequency sinusoidal electric alternating potentials in the bone region adjacent to the implant.
The transmission technique functions according to the transformer principle: the injured or ill body region is flooded by an extremely low-frequency sinusoidally extending magnetic field having a frequency of approximately 1 to 100 Hz—preferably of 4 to 20 Hz—and a magnetic flux density of 0.5 to 5 mT (5 to 50 Gauss) which is generated by a function generator in one or more—primary—outer current coils into which the body part provided with the implant is introduced. These extremely low-frequency electromagnetic fields permeate the tissue including any clothing and/or casts, as well as non-magnetic (austenitic) implant metals largely without loss. A—secondary—coil assembly, which is often referred to as a transformer, is implanted in electric contact with these. The electric potentials induced in the transformer are thus brought into effect in the tissue adjacent to the implant.
With this technology of inductively transmitting therapeutically effective electric potentials to the components of the implant, the infection risk posed by percutaneous power supply lines is avoided, and the treatment parameters of electric voltage, frequency, intensity, signal form, and treatment time can be determined by the indication-specific programming of a function generator of the induced magnetic field.
The object of the invention is to improve an orthopedic implant in the form of a generic hip joint prosthesis particularly with respect to its manageability and flexible usability during the operation, its biological effect, its therapeutic effectivity and its efficiency.
The invention is based on the generic hip joint prosthesis in that the end cap assembly carries a coil assembly connected to a first electrode with a first pole and to a second electrode with a second pole, that the first and the second electrode comprise surfaces of the hip joint prosthesis, and in a state in which the end cap assembly closes the hollow space of the hip joint prosthesis, the coil assembly is predominantly arranged in the hollow space. For this purpose, the end cap is preferably provided with an inner volume in which the pick-up coil wound around an iron core is located. It is usefully glued into the end cap assembly. Here, the ends of the coil assembly suitably contact the electrically conductive surfaces forming tissue electrodes. Based upon the invention, the end cap thus obtains a double function. On the one hand, it prevents the ingrowth of connective tissue and bones into the prosthesis which could render a later explantation difficult. On the other hand, the end cap accommodates the components which give the hip joint prosthesis electrical properties. Aside from the mentioned advantages relating to the use of a hip joint prosthesis which—with respect to the mechanical properties—is largely unchanged as compared to the state of the art, it should further be noted that the surgeon may decide whether the prosthesis is to be closed using a conventional end cap or an end cap provided with electric components during the operation. Furthermore, the provision and storage of magnetically inducible end caps are substantially less complex and, thus, more cost effective than the provision of magnetically inducible hip joint prostheses with the requisite different dimensions. Other biological advantages are as follows: the risk of an infection is reduced by enhanced blood flow and immune response of the stimulated tissue, the antibiotic resistance of the multi-resistant staphylococcus aureus (MRSA) is overcome, and the adherence of bacteria films on the surface of the prosthesis body is avoided by the magnetically induced electric activation of the surface. In the extremely low-frequency non-thermal electromagnetic field flooding the entire hip joint region, an electric field which has been proved to be bactericidally effective in in vitro trials is induced on the surface of the joint prosthesis. This also applies to the dangerous adhesion of antibiotic resistant germs, for example, in the form of biofilms. This is avoided by the electrodynamic potential on the surface of the implant. Therefore, the joint prosthetic implant is also particularly suitable to act as what is commonly referred to as a “spacer” which is implanted in a two-step operation for sterilizing and regenerating the bone in case of infected hip joint replacements and may be replaced by an individual joint prosthesis after the development of an aseptic, load-bearing bone.
It is particularly preferred that a neck section proximally connected to the shaft-like section, carrying a femoral head and comprising at least a part of the hollow space is provided, wherein the hollow space permits the insertion of fixation means and of a tool for fixing the neck section to the shaft-like section, and that the end cap assembly proximally closes the hollow space of the neck section. The hollow space of the neck section thus obtains a double function. On the one hand, it enables the insertion of fixation means, in particular, a screw, and of a tool for fixing the neck section to the shaft-like section. On the other hand, it also offers sufficient space to fully or almost fully accommodate the end cap assembly inside the implant. The outer dimensions of the implant are kept small in this way, as the electrification does not require enlargement of the implant.
According to one variant, it is contemplated that the first electrode is formed by a contact surface of the end cap assembly and that the second electrode is formed by a contact surface of the shaft-like section. In this and other embodiments, the end cap assembly may be fully or partly formed of an insulating ceramic material, wherein an electrically conductive surface area may be realized by applying an electrically conductive material.
It is also possible that the first electrode is formed by a contact surface of the end cap assembly and that the second electrode is formed by a contact surface of the neck section. This arrangement of the electrodes is particularly useful if, for any reason, the propagation of the electric field in the area of the shaft-like section is to be avoided.
However, it is also possible and, in many situations, preferred that the first electrode is formed by a contact surface of the end cap assembly and that the second electrode is formed by a contact surface of the shaft-like section and of the neck section. If the neck section electrically contacts the shaft-like section, the entirety of shaft section and neck section may form one electrode, while the other electrode is formed by a surface of the end cap assembly.
It may further be contemplated that the first electrode is formed by a contact surface of the neck section, and that the second electrode is formed by a contact surface of the shaft-like section. Therefore, the end cap is not part of the electrode assembly, but instead, the electrodes are formed by the neck section and the shaft-like section.
According to a particularly preferred embodiment of the present invention, it is contemplated that the neck section is connected to the shaft-like section by means of an axially arranged, electrically insulating screw. In this way, the two sections of the implant, i.e., the shaft-like section and the neck section, may remain insulated from each other and form the pair of counter electrodes on this basis. For example, a zirconium oxide (ZrO2) screw may be used.
In a further development of the invention, it is contemplated that a drug reservoir containing a drug is located in the hollow space and that the hollow space is connected to an outer section of the hip joint prosthesis via openings which enable a passage of the drug from the hollow space to the outer section.
According to a preferred embodiment, it is contemplated that the drug reservoir contains an antibiotic.
In this case, it has proved to be advantageous that the drug is flowable. In this way, the device according to the invention thus enables local antibiosis by activating thin antibiotics solutions on the surface of the implant and in the directly adjacent bone and soft tissue. This is particularly advantageous as antibiosis can be activated by the electromagnetic fields applied during the therapy.
The drug-related aspect of the invention can, for example, be implemented so that the drug reservoir is at least partly formed by a pressure-permeable container which at least partly releases its content when the hollow space is closed by the end cap assembly.
For example, it is possible that the end cap assembly carries the pressure-permeable container.
Prior to implantation, a ring-shaped tube filled with an antibiotic is introduced into the hollow space of the neck section or is carried by the end cap assembly, for example by being plugged into a tapered end of the end cap assembly. A thin antibiotic solution is located in the pressure permeable tube. The wall of the ring-shaped tube consists of a thin elastic plastic material which the solution can permeate under light pressure during insertion of the end cap, i.e., particularly when the end cap is screwed in.
It is also feasible that the drug reservoir cooperates with a controllable device, wherein the drug may be at least partly delivered by actuating the device. The drug, particularly the antibiotic, may, in this way, also be delivered in a targeted manner at any desired time. Thus, the delivery is not limited to that at the time of the implantation.
For example, it may be contemplated that the controllable device is suitable to directly or indirectly convert a magnetic impulse generated outside of the body into mechanical energy which effects delivery of the drug. A magnetic impulse may, for example, be converted into mechanical energy under the involvement of permanent magnetic or ferro-magnetic movable components so that the release of the drug is effected. Aside from this mechanical energy directly generated by the magnetic impulse, it is also possible that the magnetic impulse is detected by a device and then converted into a signal for generating mechanical energy of an actuator by circuit technology.
It is also feasible that the controllable device comprises an electric power storage, a receiver, an electric control circuit, and an actuator drivable by the electric control circuit so that the delivery of the drug can be effected by a signal generated outside of the body. The drivable actuator may, in particular, be a component of a drug pump.
It is particularly advantageous that the electric power storage is an accumulator chargeable with an electric voltage induced in the coil assembly. In this way, the variant comprising the electric power storage is also suitable for long-term-applications since the accumulator is chargeable at any time by means of a magnetic field applied from the outside which is deployed anyway within the course of the therapy, and which, in this respect, may provide the power for driving the drug pump.
The invention will now be described by way of example with reference to the accompanying drawings using preferred embodiments.